Would that be a RAID1 array + hot plug spare?

By the sounds of it this is waht I want (from Wikipedia):

Alternatively, a new disk can be substituted so that the inactive disk can be kept in much the same way as traditional backup. To keep redundancy during the backup process, some controllers support adding a third disk to an active pair. After a rebuild to the third disk completes, it is made inactive and backed up as described above.

Not sure how to make that happen here, but it sounds like what I'm after.